CMOS BATTERY: theres a thread on this in the Hardware mods section... it's actually turning out to be pretty easy to do
PS2/serial/parallel connectors: the port the modchip plugs into is capable of handeling this when you properly use a super I/O chip. the software needs to support it too (so no you wont be able to use keyboard/mouse in your xbox games this way ever) as far as serial and parallel go that essentially what these modchips are adding when the allow LCD support... but again the software needs to support the devices. (I personally think a daughterboard with PS2, serial, and parallel ports would be awesome)
USB: the xbox already has 4 of these and if you're using actual USB devices you can always add USB hubs for more devices.
What I'd like to see: I've wanted to see LCDs in the xbox for quite some time now and it's looking to be finally happening.
Another item I'd love too see is just a simple set of I/O points: this would allow for a wide variety of add ons like LEDs controlled by something in the software or fans controlled by the software. for instance if you made an I/O device that evox supported you could do something like when the CPU temp reaches a specified point it would turn on a Red LED and boost the fan to full speed, until it went below that temp. or you could add push buttons that could do different things
say you had 4 push buttons 1 could automatically start XBMP 1 could automatically start halo... heck maybe even make a push button with a program that checks your MP3 database on your Desktop against the collection on your Xbox and syncs up the changes. another idea would be a single button to back a game up to your HDD and then start playing it or something... with outputs from this I/o (and XBMP support) you could configure an array of LEDs controlled by the music you're playing to have a real life visual effect based on the music and not just something on the screen... there really are a lot of possibilities
with an I/O device like that it'd become even more usefull to have an LCD (espeically for audio media stuff)
then again a parallelport/serial port is essentially just an I/O port